Interferometer arrangement and interferometric measuring method

ABSTRACT

The invention relates to an interferometer arrangement ( 1000 ) having at least one measuring beam path for providing light in an object area, at least one reference beam path and at least one unit for superposing light of the measuring beam path with light of the reference beam path and a unit for detecting an interference phenomenon caused by light from the object area and light from the reference beam path. The invention further relates to a method for measuring the velocity of an object with an interferometer arrangement. In the interferometer arrangement, the unit for detecting has a spatial resolution which corresponds to the characteristic spatial period of the interference phenomenon. In evaluating the time change of the interference signal, the movement of stray centers can be measured. Use of such an interferometer arrangement in a surgical microscope allows to visualize areas of a field of surgery which cannot be accessed with light in the visible spectrum.

FIELD OF THE INVENTION

The invention relates to an interferometer arrangement and to aninterferometric measuring method for measuring the velocity of anobject. The object is illuminated with a measuring beam. The light whichis reflected from the object interferes with light of a reference beampath thereby generating an interference phenomenon pattern. Theinterference phenomenon pattern is detected with a detector and istransmitted to an evaluation unit.

BACKGROUND OF THE INVENTION

U.S. Pat. No. 6,396,587 is incorporated herein by reference anddiscloses an interferometer arrangement for recording depth profiles ofan object using “white light interferometry”.

In white light interferometry, white light, that is light having arelatively short coherence length, is used. The coherence length of anoptical signal is the length over which the signal is phase correlated.For a light source having a long coherence length as, for example, ahelium-neon laser, the coherence length can be several kilometers, whilefor a broadband white light source as, for example the sun, thecoherence length is only some micrometers. Light sources having such ashort coherence length cause interference phenomena in a light beampath, which is split and then again superposed, only if the optical pathlength of the two light beam paths between splitting and superpositioncorresponds within a few optical path lengths, that is, if the opticalpath lengths correspond within the coherence length.

The interferometer arrangement disclosed in U.S. Pat. No. 6,396,587includes a superluminescence diode for generating light, which can beused for white light interferometry. The light of this superluminescencediode is coupled into a measuring beam path and a reference beam path.The reference beam path includes a mirror as a reflector element. Withthe measuring beam path, an object is scanned for measuring its depthprofile and/or the density of scattering light centers. The lightreflected from the object and from the mirror is superposed on a sensorsurface which includes a CCD-detector row. From the location of aninterference pattern on the CCD-detector row, a difference in theoptical path length of the light in the measuring beam path and thereference beam path can be determined. From the measured difference ofthe optical path lengths, the position of scattering centers for lightin the scanned or depth-probed object is determined and graphicallyrepresented.

U.S. Pat. No. 5,321,501 is incorporated herein by reference anddiscloses an interferometer arrangement and a method for determiningdepth profiles of an object using short coherent light from asuperluminescence diode. The short coherent light from thesuperluminescence diode is coupled into a measuring beam path and areference beam path. With the light in the measuring beam path, asection of an object is scanned or depth probed. The light in thereference beam path is guided to a movable mirror which allows to adjustthe optical path length in the reference beam path. The light of themeasuring beam path and of the reference beam path are brought tointerference on a photodetector. From the time variation of subsequentinterference phenomena on the photodetector and the known position ofthe mirror in the reference beam path, the position of scatteringcenters in the measuring branch is determined.

SUMMARY OF THE INVENTION

It is an object of the invention to provide an interferometerarrangement having a very high measuring accuracy. It is a furtherobject of the invention to provide a method for measuring the velocityof an object with an interferometer.

The interferometer arrangement of the invention includes a measuringbeam path directed to and reflected from a specimen or object and areference beam path. The interferometer has means for superposing lightof the measuring beam path on light of the reference beam path togenerate an interference phenomenon pattern having a characteristicspatial frequency. The interferometer arrangement further includes meansfor detecting the interference phenomenon pattern. The detecting meanshas a minimum spatial resolution which corresponds to the characteristicspatial frequency of the interference phenomenon pattern. Theinterference phenomenon pattern manifests itself in a spatial variationof the intensity of light at the detecting means. In the presentinvention, the characteristic spatial frequency of the interferencephenomenon pattern matches the spatial resolution of the detectingmeans. This is achieved in that the detecting means can resolve thespatial period of the interference pattern. In this way, aparameter-optimized interferometer is provided which has a relativelylow signal to noise ratio and allows measuring distances reliably basedon comparatively little data.

According to a feature of the invention, the detecting means includes aCCD-detector row having a detector pixel geometry which is adapted tothe characteristic spatial frequency so that the period of theinterference phenomenon covers at least two of the detector pixels. Withthis feature, the number of pixels of the CCD-row can be kept low. Thismakes possible high read-out rates of the CCD-row and allows for simplerdata processing because the amount of data is correspondingly small.

According to another feature of the invention, the means for superposinglight of the measuring beam path onto light of the reference beam pathinclude optics for focusing the light of the measuring beam path and thelight of the reference beam path onto the detecting means. The opticshas an aperture which is adapted to the detecting means so that all thelight of the measuring beam path and the reference beam path is detectedon the detecting means. Since, in this way, all the light carryinginterference phenomenon information can be evaluated on the detectorrow, a good signal-to-noise ratio can be achieved.

According to another feature of the invention, the optics for focusingthe light of the measuring beam path and the reference beam path on thedetecting means illuminate an area on the detecting means which measurestwice the radius of a spatial Gaussian intensity profile of lightfocused on the detecting means. This means that the intensity of lightfrom the measuring beam path and the reference beam path interfering onthe CCD-row is at least 1/e² of the maximum intensity of the light onthe CCD-row. In this way, the CCD-array is illuminated in a way which isadapted to the measuring range of the detector pixels on the CCD-row.

Preferably, the interferometer arrangement includes a first lightconductor arranged in the measuring beam path and a second lightconductor arranged in the reference beam path. The first which isadapted to the characteristic spatial frequency so that the period ofthe interference phenomenon covers at least two of the detector pixels.With this feature, the number of pixels of the CCD-row can be kept low.This makes possible high read-out rates of the CCD-row and allows forsimpler data processing because the amount of data is correspondinglysmall.

According to another feature of the invention, the means for superposinglight of the measuring beam path onto light of the reference beam pathinclude optics for focusing the light of the measuring beam path and thelight of the reference beam path onto the detecting means. The opticshas an aperture which is adapted to the detecting means so that all thelight of the measuring beam path and the reference beam path is detectedon the detecting means. Since, in this way, all the light carryinginterference phenomenon information can be evaluated on the detectorrow, a good signal to noise ratio can be achieved.

According to another feature of the invention, the optics for focusingthe light of the measuring beam path and the reference beam path on thedetecting means illuminate an area on the detecting means which measurestwice the radius of a spatial Gaussian intensity profile of lightfocused on the detecting means. This means that the intensity of lightfrom the measuring beam path and the reference beam path interfering onthe CCD-row is at least 1/e² of the maximum intensity of the light onthe CCD-row. In this way, the CCD-array is illuminated in a way which isadapted to the measuring range of the detector pixels on the CCD-row.

Preferably, the interferometer arrangement includes a first lightconductor arranged in the measuring beam path and a second lightconductor arranged in the reference beam path. The first light conductorand the second light conductor have end sections where light exits forgenerating the interference phenomenon pattern. The detecting meansinclude CCD-detector pixels of size Δx. The end sections are at adistance 2 a from each other and a distance d from the detecting means.Δx, 2 a, d, and the wavelength of light λ used in the interferometerarrangement satisfy the following relationship:$\frac{\lambda\; d}{{2a\;\Delta\; x}\;} \geq 2.$

In this way, the sensitivity of the interferometer arrangement isoptimized for a given geometry of the pixels on the CCD-row.

According to another feature of the invention, the interferometerarrangement includes a filter unit for filtering the interferencephenomenon pattern. This allows for detecting a difference in theoptical path lengths between the measuring beam path and the referencebeam path even if the reflection properties of the object which isprobed are of inferior quality.

Preferably, the filter unit includes a heterodyne filter having a filterfrequency which corresponds to the characteristic spatial frequency ofthe interference phenomenon pattern. In this way, a very narrow bandpassfiltering of the interference phenomenon is achieved and interferencephenomena can be detected at a low signal-to-noise ratio. Especially, aninterferometer arrangement which includes such a filter unit can resolvethe position of scattering centers having a very low reflectivity.

In another embodiment of the invention, the interferometer arrangementincludes a signal evaluation unit which is coupled to the means fordetecting the interference phenomenon to determine a time variation ofthe interference phenomenon pattern. This allows to measure preciselythe movement of scattering centers in the object region exposed to themeasuring beam.

According to another feature of the invention, two reference beam pathsare provided in the interferometer arrangement. In this way, adifference in the optical path length for light reflected in the area ofthe object subjected to the measuring beam can be compared to theoptical path lengths of the reference beams so that a statisticalmeasuring error can be minimized and an enhanced resolution or accuracyof the interferometer arrangement can be achieved.

Preferably, the two reference beam paths in the interferometerarrangement have different optical path lengths. In adjusting themeasuring ranges of the corresponding reference beam paths, the dynamicrange of the interferometer arrangement can be increased. Compared to aninterferometer having only one reference beam, the measuring range isexpanded by the measuring range of the other reference beam path.

According to another feature of the invention, the first reference beampath and the second reference beam path and the measuring beam path arearranged to generate an interference phenomenon which is characterizedby two different characteristic spatial frequencies, the ratio of whichis neither an integer nor an integer ratio. In this way, a goodseparation of signals which are superposed on a single CCD-row can beachieved using heterodyne filtering. Here, use is made of the fact thatin filter function regimes, which correspond to higher harmonics of onecharacteristic signal frequency, there is no signal which is related tointerference phenomena of a different characteristic basic frequencycaused by the other reference beam path.

A surgical microscope, which includes such an interferometerarrangement, can provide three-dimensional sectional views of an objectfor a surgeon.

In monitoring the time variation of an interference pattern, thedetecting means determines the velocity of an object in the measuringbeam path. In this way, the flow of a medium in a capillary or a smalltube can be measured.

In focusing the light of the probe beam path on the object, the flow ina capillary can be spatially resolved. This allows, for example, tomeasure the local flow profile of blood in arteries.

If a CCD-detector row is used for detecting the interference pattern andthe time variation of the interference pattern is determined bycomparing sequential images on the CCD-detector row, the movement ofscattering centers in a cross section of the region of the object orspecimen exposed to the measuring beam of the interferometer arrangementcan be determined with very high accuracy.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will now be described with reference to the drawingswherein:

FIG. 1 is a schematic representation of a first embodiment of theinterferometer arrangement;

FIG. 2 is a section of the interferometer arrangement shown in FIG. 1;

FIG. 3 illustrates an interferometer phenomenon pattern which isdetected with the CCD-row of the interferometer arrangement shown inFIG. 1;

FIG. 4 shows a distance information signal based on two reflectingsurfaces measured with the interferometer shown in FIG. 1;

FIG. 5 shows a probe section of the interferometer arrangement of FIG. 1and a probe having an inhomogeneous distribution of scattering centers;

FIG. 6 shows an interferometer arrangement signal illustrating theinformation gained on the position of scattering centers for the probeshown in FIG. 5;

FIG. 7 shows a section of the interferometer arrangement presented inFIG. 1 and a liquid or gas flowing through a capillary;

FIG. 8 illustrates the flux profile of a laminar flow in the capillaryshown in FIG. 7;

FIG. 9 illustrates the information on the flux profile in form of atime-dependent phase shift as determined with the interferometerarrangement shown in FIG. 1;

FIG. 10 is a schematic representation of a second embodiment of theinterferometer arrangement having a greater dynamic range;

FIG. 11 shows a section of the interferometer arrangement presented inFIG. 10; and,

FIG. 12 illustrates an interference phenomenon pattern on the CCD-row ofthe interferometer arrangement presented in FIG. 10.

DESCRIPTION OF THE PREFERRED EMBODIMENTS OF THE INVENTION

FIG. 1 shows an interferometer arrangement 100 which includes a sourceof white light 101 in the form of a superluminescence diode. The sourceof white light 101 emits luminescence light in a wavelength domain of850 nm with a coherence length of about 10 nm. It is understood, thatother sources of white light, which emit light in other spectraldomains, could also be used. The source of white light 101 provideslight for a measuring beam path and a reference beam path. For this, thelight from the source of white light 101 is coupled in a light guide102, in which a light coupling unit 103 is arranged. In this lightcoupling unit 103, the light from the source of white light 101 is mixedat a ratio of 90:10 with the light from a laser diode 104, which emitslight in the visible spectrum. With the visible light from the laserdiode 104, an adjusting beam is provided for the measuring area of theinterferometer arrangement 100. The white light, which is mixed with thevisible laser light, is guided through a light guide 105 to anotherlight coupling unit 106. This light coupling unit 106 couples light in ameasuring beam path which includes a light guide 107 and a referencebeam path which includes a light guide 108. The light coupling unit 106distributes the light from the light guide 105 in the measuring beampath and the reference beam path at a ratio of 90:10. The light couplingunit 106 functions as turnout or switch for light and guides 90% of thelight from light guide 105 to the light guide 107 and guides 10% of thelight from the light guide 105 to the light guide 108.

In the reference beam path, another light coupling unit 109 is arranged,which provides light for a reference beam area via a light guide 110 anda lens system 111. This reference beam area in the reference beam pathincludes a mirror 112 which is exposed to the light from light guide110. The light reflected on the mirror 112 is coupled back into thelight guide 110 and guided back to the light coupling unit 109. Thelight coupling unit 109 diverts 50% of the light provided by the lightguide 110 into the light guide 113 which guides it to an arrangement fordetecting an interference phenomenon pattern caused by light reflectedfrom an object area and the reference beam area. The arrangement fordetecting the interference phenomenon pattern includes as a detectorunit a CCD-row 117. The light exiting the light guide 113 at its exitend 114 is guided through a cylindrical lens 115 to generate a raybundle 116 on this CCD-row 117.

The light, which is coupled into the measuring beam path by the lightcoupling unit 106, is conducted via a light coupling unit 118 and alight guide 119 and is focused on the object area of an object 121,which is examined, by an optical imaging system 120.

The light reflected from the object 121 is guided back through theoptical imaging system 120 and coupled into the light guide 119, whichconducts the light to the light coupling unit 118. This light couplingunit 118 couples 50% of the light from light guide 119 into the lightguide 122.

In a manner corresponding to the light of light guide 113, the light oflight guide 122 exits the light guide at an end 123, passes thecylindrical lens 115 to form a ray bundle 124 reaching the CCD-row 117.With the CCD-row 117, an interference phenomenon pattern is detected,which is caused by the interference of light from the object area withlight reflected on the mirror 112 if the conditions for interference aremet.

The CCD-row 117 is coupled to an interference signal evaluation unit130. This interference signal evaluation unit 130 includes a filterstage 131 which is connected to an oscillation signal detection unit132.

The interferometer arrangement further includes a scan unit 140, whichallows for two-dimensional scanning of the object 121 to be examined.From the scanning position of the scan unit and by evaluating theinterference phenomena on the CCD-row 117, a distribution of scatteringcenters in the object area can be visualized as a three-dimensionalimage.

FIG. 2 shows a section 200 of the interferometer arrangement presentedin FIG. 1. Referring to FIG. 1, this section includes the ends of thelight guides 114 and 123 as well as the cylindrical lens 115 with theCCD-row 117.

Referring to FIG. 2, the light from the light guides 201 and 202traverses the cylindrical lens 203 to be focused to a narrow line on theCCD-row 204. This narrow line is detected by the light-sensitivedetector pixels of the CCD-row 204. The light exiting the light guides201 and 202 has a Gaussian intensity profile. This causes a resultinglight intensity distribution I on the CCD-row 204 characterized by anenvelope which is the superposition of two Gaussian intensitydistributions.

Assuming that the reflectance of the object area 121 in theinterferometer shown in FIG. 1 and of the reference mirror 112 is thesame, the following holds for the intensity distribution I on theCCD-row 117 in dependence on the position 205 with the origin of thecoordinate system at the location indicated by reference numeral 206 inFIG. 2:${{I(x)} = {2{I_{0}\left( {1 + {V_{x}{\cos\left( {{2\;\pi\frac{c}{\lambda}\left( {\tau_{x} + {\Delta\tau}_{z}} \right)} + \varphi_{x}} \right)}}} \right)}}},$wherein:

-   λ is the average wavelength of the light from the source of white    light;-   c is the velocity of light; $\tau_{x} = \frac{\theta\; x}{c}$    is, for different locations x on the CCD-row, the difference in    travel time for the light from the two light guides 201 and 202    defining the opening angle ${\theta = \frac{a}{2d}};$-   Δτ_(z) is the difference in travel time for the light of the two    light guides 201 and 202 because of the different optical path    lengths of the measuring beam path and the reference beam path for    the interferometer arrangement shown in FIG. 1;-   V_(x) is the envelope of the interference phenomenon pattern in the    form of a Gaussian distribution having a half width    $x_{c} = \frac{L_{c}}{\theta}$     for a coherence length L_(c) of the light from the source of white    light used; and,-   φ_(x) is the phase factor having only a relatively minor dependence    on location as compared to the oscillatory component of the    interference phenomenon.

FIG. 3 illustrates on an enlarged scale an intensity distribution 300with an interference phenomenon pattern 301 at location x_(i) on theCCD-row 117 of FIG. 1. The interference phenomenon pattern 301 has acharacteristic frequency: $k_{c} = {2\;\pi\frac{\theta}{\lambda}}$and extends at position x_(i) over a width$x_{c} = {\frac{L_{c}}{\theta}.}$

The location x_(i) of the interference phenomenon pattern 301 on theCCD-row 117 of the interferometer arrangement 100 shown in FIG. 1 thusis determined by the difference in the optical path lengths for thelight of the source of white light in the corresponding measuring beampath and the reference beam path. The location x_(i) corresponds to awell defined difference in the optical path length of the reference beampath and a ray bundle reflected from a scattering center in themeasuring beam path. By evaluating the position of the interferencephenomenon pattern 301 in FIG. 3 on the CCD-row 117 of theinterferometer arrangement 100 of FIG. 1, the exact position of ascattering center in the object area can be measured. Furthermore, fromthe time variation of the interference phenomenon pattern on theCCD-row, the state of movement of the scattering centers in the objectarea can be determined. This means that it is possible to measure, in aspatially resolved manner, the velocity of these scattering centers.

In the interferometer arrangement 100 of FIG. 1, the means for detectingthe interference phenomenon having the CCD-row 117 and the cylindricallens 115 is adapted to the characteristic frequency of the interferencephenomenon. For this purpose and referring to FIG. 2, the mutualdistance 2 a of the ends of the light guides 201 and 202 as well as thedistance d of the ends of the light guides from the CCD-row 204corresponds to the distance and the size of the detector pixels on theCCD-row so that the spatial variation of a light intensity signal on theCCD-row 117 can be resolved at the characteristic frequency. Hereby,frequency components of this light intensity signal, which are greaterthan the characteristic frequency, are not resolved. This allows todetect a variation of the light intensity on the CCD-row 204 having aspatial frequency which is less than the characteristic frequency whilereading out the CCD-row 204. At the same time, variations of the lightintensity on the CCD-row 204, which occur at higher frequencies than thecharacteristic frequency (for example, because of disturbing effects),are averaged out.

The area 205 on the CCD-row 204 is illuminated both by light from thelight guide 201 and by light from the light guide 202 so that thelight-sensitive section of the CCD-row 204 is exposed to the light raysfrom the light guides 201 and 202, which reach the CCD-row 204 withintwice the Gaussian radius of the corresponding intensity profile of thelight which comes from the light guides 201 and 202. It should be notedthat, in principle, it is also possible to illuminate the CCD-row 204with light from the light guides 201 and 202 at a different intensityprofile. For example, the CCD-row 204 could only be illuminated up toonce the Gaussian radius. This means that the resulting intensity of thelight, which reaches the CCD-row 204, corresponds at least to 1/e of themaximum light intensity on the CCD-row 204.

The measuring range of the interferometer arrangement is determined bythe size of the CCD-row section, which is illuminated and on which lightfrom the measuring beam path and the reference beam path can interfere.Assuming that in this section there are N pixels in the CCD-row and thatthe spatial period P_(c) of an interference signal having thecharacteristic frequency k_(c) covers P pixels, the following holds forthe measuring range Δz of the interferometer arrangement while usingwhite light of a source of white light having an average wavelength λ:${\Delta\; z} = {\frac{N\;\lambda}{2P}.}$

This means that the position of scattering centers in the object areacan only be measured over a depth Δz.

In the interferometer arrangement 100 shown in FIG. 1, the sizes and therespective spacings of the detector pixels of the CCD-row 117 are soselected that the intensity variations of interference light, whichoccur at the period of the characteristic frequency k_(c), cover atleast two detector pixels. In this way, a maximum measuring range can beachieved.

To detect a position information of a scattering center in the objectarea, the CCD-row 117 of FIG. 1 is sequentially read out and theintensity information of each CCD-pixel on the CCD-row is converted intoa voltage value. With this sequential serial readout process, atime-dependent electrical voltage signal is generated. By filtering thisvoltage signal, the disturbances of the interference signal can beremoved and the position information carried by the interference signalon the CCD-row can be determined.

For this purpose, the time-dependent voltage signal is filtered in thefilter stage 131 of the interference signal evaluation unit 130 of theinterferometer arrangement of FIG. 1 with a narrow bandpass filter at atime frequency which corresponds to the characteristic spatial frequencyk_(c). The filter unit 131 is formed by a heterodyne filter having afilter frequency, which corresponds to the characteristic frequencyk_(c). Such a heterodyne filter has a very narrow filter function. As analternative, other methods of filtering could be used, as for example,LC-filter arrangements or the like.

In the oscillation signal detecting unit 132, which follows the filterunit 131 in the interferometer arrangement 100, the envelope of thefiltered voltage signal is determined. The magnitude of this voltagesignal at a certain time point is a measure for position and size of ascattering center in the object area.

Furthermore, in the oscillation signal detecting unit 132, the phaseposition of the filtered voltage signal relative to a reference value isdetermined. For this, the time points of zero crossovers of the filteredvoltage signal are determined. By following the phase position of suchan interference phenomenon in sequential read outs of the CCD-row, aconclusion can be drawn as to the movement of a scattering center in theobject area of the interferometer arrangement 100 of FIG. 1 bydetermining the phase position of the filtered voltage signal.

In principle, any object can be investigated with the interferometerarrangement, provided that they at least partially reflect light whichis directed onto them. Such objects may be biological material, forexample, portions of a human or animal body or plants. It should benoted, however, that the interferometer arrangement may also be used tomeasure workpieces in an industrial production process, for example,workpieces in a machine tool where dimensions have to be determined veryaccurately. The interferometer arrangement can also be used to measurethe dimensions of optical lenses.

FIG. 4 shows a distance signal 400, which is based on a voltage signalgenerated by reading out the CCD-row 117 in the interferometerarrangement 100 of FIG. 1. The time-dependent envelope I, which isdetermined in the oscillation signal detection unit 132 of theinterferometer arrangement 100, is plotted with its position informationcontent as a function of the position z, which corresponds to a certainposition of scattering centers. This signal comes from an investigatedobject which has two spatially displaced reflection surfaces.

FIG. 5 shows a section of the interferometer arrangement 100 of FIG. 1with a sample 500 including a portion 501 having a first homogeneousdensity of scattering centers and a second portion 502 having anothersecond homogeneous density of scattering centers. This sample is scannedor depth probed with the optical imaging system 503.

In the interferometer arrangement, the scattering centers generate anintensity distribution signal which is shown in FIG. 6. This intensitydistribution signal can be processed for image generation whereby adensity distribution of scattering centers is assigned to the structureof the signal intensity distribution 600 shown in FIG. 6.

FIG. 7 illustrates how the velocity profile of a fluid flowing through acapillary can be measured. The interfering light of the light guide 701is focused on optical scattering centers in the fluid flow 703 in thecapillary 704. If the scattering centers pass the focus 705 in theobject area of the interferometer, they cause the interference patternon the CCD-row to travel. This is because the light reflected from thescattering centers in the fluid flow interferes with the light in thereference beam path while the differences in the optical path lengthsbetween the measuring beam path and the reference beam path arecontinuously changed as the scattering centers move. From the timevariation of the phase of the corresponding interference signal on theCCD-row 117, the interference signal evaluation unit 130 determines amovement of the scattering centers. The measurement of the velocity isthe more precise the smaller the opening angle of the motion directionof the scattering centers to the direction of the scanning light.

The flow profile 800 of a laminar flow in a capillary having a radius r₀is shown in FIG. 8. The flow profile 800 has a maximum at the center ofthe flow. At the borders of the capillary however, the flow of the fluidis zero.

FIG. 9 shows the rate of change φ of the phase of the filteredoscillation signal in dependence on the position d of the scatteringcenters in the object area when the flow profile of the fluid in thecapillary is monitored with the interferometer arrangement 100 shown inFIG. 1 as explained with respect to FIG. 7. The position-dependent rateof change 900 corresponds to a local flow velocity of the scatteringcenters. This position dependent rate of change mirrors the flow profileof a laminar fluid flow as shown in FIG. 8.

FIG. 10 shows an interferometer arrangement 1000, which has a greatermeasuring range than the interferometer arrangement 100 shown in FIG. 1.The interferometer arrangement 1000 includes a source of white light1001 which provides light that is coupled into light guide 1002, as inthe interferometer arrangement 100 of FIG. 1. The light of the lightguide 1002 is supplied to a light coupling unit 1003, where it is mixedat a ratio of 90:10 with light from a laser diode 1004 providing lightin the visible spectrum. With the visible light of the laser diode 1004,an adjusting laser beam is provided for the object area of theinterferometer arrangement 1000. The mixed light is supplied via a lightguide 1005 to a light coupling unit 1006, where it is divided at a ratioof 90:10 between a measuring beam path which includes a light guide 1007and a reference beam path having a light guide 1008. The light couplingunit 1009 gives light to a light guide 1010, which guides referencelight in a reference beam path for interference. The light from thelight guide 1010 is guided to a mirror 1012 via a lens system 1011. Thelight reflected back from this mirror 1012 is guided back via the lightguide 1010 to the light coupling unit 1009. This light coupling unit1009 couples 50% of the light of light guide 1010 into a light guide1013. In the light guide 1013, another light coupling unit 1014 isarranged, which divides the light supplied by light guide 1013 at aratio of 50:50 between a light guide 1015 and a light guide 1016. Thelight guides 1015 and 1016 have ends 1017 and 1018, respectively, wherelight ray bundles 1019 and 1020 exit with a Gaussian beam profile. Theseray bundles 1019 and 1020 are focused to a narrow line on the CCD-row1022 by a cylindrical lens 1021.

The light coupled into the measuring beam path by the light guide 1007is conducted to a light coupling unit 1023 which is connected to a lightguide 1024 and an optical imaging system 1025. This optical imagingsystem 1025 includes two lenses, which are provided for focusing ameasuring light ray bundle onto the object area of an object 1026 whichis examined. The light reflected from the object 1026 again enters thelight guide 1024 and is guided back to the light coupling unit 1023where it is mixed with the light from the measuring beam path 1007 sothat 50% of the light from the light guide 1024 is passed to the lightguide 1027. The light guide 1027 has an end 1028 where a ray bundle 1029having a Gaussian beam profile exits. This ray bundle 1029 is focused onthe CCD-row 1022 by the cylindrical lens 1021 to a narrow line.

Analog to the interferometer arrangement 100 of FIG. 1, theinterferometer arrangement 1000 includes an interference signalevaluation unit 1030 having a filter stage 1031 and an oscillationsignal detection unit 1032.

FIG. 11 shows a portion 1100 of the interferometer arrangement 1000 ofFIG. 10. In FIG. 11, the end sections of the light guides 1017, 1018 and1028 with the cylindrical lens 1021 and the CCD-row 1022 of FIG. 10 areshown at an enlarged scale.

The light of the light guides 1101, 1102 and 1103 is focused to a narrowline on the CCD-row 1105 by the cylindrical lens 1104. The section 1106of the CCD-row 1105 is illuminated simultaneously by light from thelight guides 1101, 1102 and 1103 so that the minimum intensity of thelight of each light guide lies within the intensity region correspondingto twice the Gaussian radius of the intensity profile. As explained withrespect to FIG. 2, however, it is also possible to illuminate theCCD-row according to some other criterion for the intensity.

In this way, interference phenomena in the section 1106 can be detectedwith the CCD-row 1105. Because of the different distance a₁₂ of the exitend of the light guide 1101 from the exit end of light guide 1102 andthe distance a₁₃ of the exit end of light guide 1101 from the exit endof light guide 1103, interference phenomenon patterns occur because ofthe superpositions of light from light guide 1101 upon light from lightguide 1102 and the patterns have the characteristic frequencies$k_{c1} = {2\pi\frac{a_{12}}{\lambda\; 2d}}$ and${k_{c2} = {2\pi\frac{a_{13}}{\lambda\; 2\; d}}},$wherein:

-   d is the distance of the exit ends of the light guides 1101, 1102    and 1103 from the CCD-row; and,-   λ is the wavelength of the source of white light 1001.

The two different characteristic frequencies k_(c1) and k_(c2) of theinterference phenomena caused by light from the corresponding lightguides permit these phenomena to be separated from each other and toassign each interference phenomenon a corresponding difference of theoptical path length of the measuring beam path and the reference beampath.

FIG. 12 illustrates, on an enlarged scale, the light intensitydistribution 1200 for two scattering centers at different positions inthe object area 1026 which results on the CCD-row of an interferometerarrangement 1000 as shown in FIG. 10. The light intensity distribution1200 includes an interference phenomenon pattern 1201 at location x_(i1)having a characteristic frequency k_(c1) and an interference phenomenonpattern 1202 at location x_(i2) having a characteristic frequencyk_(c2).

The spatial resolution of the means for detecting the interferencephenomena pattern is adapted to these characteristic frequencies k_(c1)and k_(c2) as in the interferometer 1000 of FIG. 10. Therefore, themutual distances a₁₂ and a₁₃ of the exits of the light guides 1101, 1102and 1103 as well as the distance d of the CCD-row from those exits isadapted to the size of the detector pixels on the CCD-row so that thecharacteristic frequencies k_(c1) and k_(c2) can be resolved and theoscillatory components of light, which exceed those frequencies, areaveraged out. This means, disturbing signals, which do not contain anyposition information as to scattering centers, are not detected. Inorder to have a wide measuring range, the size and the distance of thedetector pixels are selected in a way that the period of the greatercharacteristic frequency covers at least two detector pixels.

For measuring the position of a scattering center in the object area,the CCD-row 1022 of FIG. 10 is sequentially read out and the intensityinformation of each CCD-pixel is converted into a voltage value. Thisserial read-out process generates a time dependent electric voltagesignal. By filtering this voltage signal, disturbing signal componentscan be removed and the position information of the voltage signal can beextracted.

For this, the time dependent electrical voltage signal is filtered inthe filter stage 1031 having narrow bandpass filter means referred totime frequencies, which correspond to the characteristic spatialfrequencies k_(c1) and k_(c2). The filter stage 1031 includes twoparallelly-connected heterodyne filters having different filterfrequencies, corresponding to the characteristic frequencies k_(c1) andk_(c2). For this reason, an especially narrow band filtering iseffected.

The amplitudes of the filtered voltage signals are determined in theoscillation signal detecting unit 1032 which follows the filter stage1031 of the interferometer arrangement 1000 of FIG. 10. The magnitudesof these voltage signals at a certain time point are a measure for theposition and dimension of a scattering center in the object area.

Furthermore, the phase position of the filtered voltage signals inrelation to a reference value is determined in the oscillation signaldetecting unit 1032. For this purpose, the time points of the zerocrossovers of the filtered voltage signals are monitored. In followingthe phase shift of an interference phenomenon in sequential read-outs ofthe CCD-row, a conclusion can be drawn as to the movement of ascattering center in the object area of the interferometer arrangement1000 of FIG. 10.

For realizing a good separation of signals of the interference phenomenaat the characteristic frequencies k_(c1) and k_(c2), the wavelength λ,the distance d of the ends of the light guides 1101, 1102 and 1103 fromthe CCD-row 1105 as well as the distances a₁₂ and a₁₃ of the ends of thelight guides 1101, 1102 and 1103 are selected so that the onecharacteristic frequency is not an integer multiple of the othercharacteristic frequency.

The interferometer 1000 of FIG. 10 further includes a delay line 1016 awhich is adapted to the coherence length of the white light used, sothat a continuous dynamic range of the interferometer arrangement isprovided, which corresponds roughly to twice the dynamic range of aninterferometer arrangement having only one reference beam path.

It is noted, that the dynamic range of the interferometer shown in FIG.10 can be expanded by adding further delay lines, in which light iscoupled in.

A surgical microscope, which includes an interferometer arrangement asexplained with respect to FIGS. 1 and 10 can make visible areas of afield of surgery with a high optical resolution which cannot be accessedwith light in the visible spectrum.

It is understood that the foregoing description is that of the preferredembodiments of the invention and that various changes and modificationsmay be made thereto without departing from the spirit and scope of theinvention as defined in the appended claims.

1. An interferometer arrangement comprising: a measuring beam pathdirected to and reflected from an object; a reference beam path; meansfor interfering light of said measuring beam path with light of saidreference beam path to generate an interference phenomenon patternhaving a characteristic spatial frequency (k_(c)); and, means fordetecting said interference phenomenon pattern so as to provide aspatial resolution matching said characteristic spatial frequency(k_(c)) of said interference phenomenon.
 2. The interferometerarrangement of claim 1, wherein said means for detecting saidinterference phenomenon includes a CCD-detector row having detectorpixels; said detector pixels having a geometry adapted to saidcharacteristic spatial frequency of said interference phenomenon so thatthe period of said interference phenomenon covers at least two of saiddetector pixels.
 3. The interferometer arrangement of claim 1, whereinsaid means for interfering light of said measuring beam path with lightof said reference beam path include optics for focusing said light ofsaid measuring beam path and said light of said reference beam path onsaid detecting means; said optics having an aperture adapted to saidmeans for detecting said interference phenomenon so that all of thelight of said measuring beam path and said reference beam path iscaptured on said detecting means.
 4. The interferometer arrangement ofclaim 1, wherein said interfering means includes optics for focusingsaid light of said measuring beam path and said light of said referencebeam path on said detecting means; and, said optics being configured toilluminate an area on said detecting means which corresponds to twicethe radius of a spatial Gaussian intensity profile of the light focusedon said detecting means.
 5. The interferometer arrangement of claim 1,further comprising: a light source providing light for said measuringbeam path and said reference beam path; a first light conductor definingsaid measuring beam path; a second light conductor defining saidreference beam path; said first and second light conductors having endsections exiting light for generating said interference phenomenonpattern; said detecting means including CCD-detector pixels of size Δx;said end sections being separated by a distance 2 a from each other anda distance d from said detecting means; and, said light used in theinterferometer arrangement being provided by said light source havingthe wavelength λ; wherein, Δx, 2 a, d and λ satisfy the followingrelation: $\frac{\lambda\; d}{2a\;\Delta\; x} \geq 2.$
 6. Theinterferometer arrangement of claim 1, further including a filter unitfor filtering said interference phenomenon pattern.
 7. Theinterferometer arrangement of claim 6, wherein said filter unit includesa heterodyne filter having a filter frequency which corresponds to saidcharacteristic spatial frequency of said interference phenomenonpattern.
 8. The interferometer arrangement of claim 1, furthercomprising: means for monitoring a time variation of said interferencephenomenon pattern; means for determining the velocity of said objectfrom said time variation of said interference phenomenon pattern; and,wherein said detecting means include a CCD-detector row for detectingsaid interference phenomenon pattern; and, said time variation of saidinterference pattern is monitored by comparing sequential images on theCCD-detector row.
 9. The interferometry arrangement of claim 1, whereinsaid detecting means include a CCD-row having detector pixels havingsizes and respective spacings therebetween so selected that intensityvariations of interference light, which intensity variations occur atthe period of said characteristic frequency (k_(c)), cover at least twoof said detector pixels.
 10. The interferometer arrangement of claim 1,further comprising: a light source providing light for said measuringbeam path and said reference beam path; a first light conductor definingsaid measuring beam path; a second light conductor defining saidreference beam path; said first and second light conductors having endsections exiting light for generating said interference phenomenonpattern; said detecting means including CCD-detector pixels of size Δx;said end sections being separated by a distance 2 a from each other anda distance d from said detecting means; and, said light used in theinterferometer arrangement being provided by said light source havingthe wavelength λ; wherein, Δx, 2 a, d and λsatisfy the followingrelation: $19.2 > \frac{\lambda\; d}{2a\;\Delta\; x} \geq 2.$
 11. Theinterferometer arrangement of claim 1, further comprising: a lightsource providing light for said measuring beam path and said referencebeam path; a first light conductor defining said measuring beam path; asecond light conductor defining said reference beam path; said first andsecond light conductors having end sections exiting light for generatingsaid interference phenomenon pattern; said detecting means includingCCD-detector pixels of size Δx; said end sections being separated by adistance 2 a from each other and a distance d from said detecting means;and, said light used in the interferometer arrangement being provided bysaid light source having the wavelength λ; wherein, Δx, 2 a, d and λdefine the quantity $\frac{\lambda\; d}{2a\;\Delta\; x}$ which isapproximately equal to
 2. 12. An interferometer arrangement comprising:a measuring beam path along which a measuring beam is directed to andreflected from a region of an object whereat scattering centers areexposed to said measuring beam; a reference beam path; means forinterfering light of said measuring beam path with light of saidreference beam path so generating an interference phenomenon patternhaving a characteristic spatial frequency; means for detecting saidinterference phenomenon pattern; and, a signal evaluation unit coupledto said detecting means for determining a variation in time of saidinterference phenomenon to permit precise measurement of movement ofsaid scattering centers in the object region exposed to said measuringbeam.
 13. An interferometer arrangement comprising: a measuring beampath directed to and reflected from a specimen; a first reference beampath; a second reference beam path; means for interfering light of saidmeasuring beam path with light of said first reference beam path andsaid second reference beam path for generating an interferencephenomenon pattern having one or more characteristic spatialfrequencies; and, means for detecting said interference phenomenonpattern.
 14. The interferometer arrangement of claim 13, wherein saidfirst reference beam path and said second reference beam path haverespective optical path lengths different from each other.
 15. Theinterferometer arrangement of claim 13, wherein said first referencebeam path, said second reference beam path and said measuring beam pathare arranged to generate an interference phenomenon characterized by twodifferent characteristic spatial periods, the ratio of saidcharacteristic spatial periods being neither an integer nor an integerratio.
 16. The interferometer arrangement of claim 13, furthercomprising: means for monitoring a time variation of said interferencephenomenon pattern; means for determining the velocity of said objectfrom said time variation of said interference phenomenon pattern; and,wherein said detecting means include a CCD-detector row for detectingsaid interference phenomenon pattern; and, said time variation of saidinterference pattern is monitored by comparing sequential images on theCCD-detector row.
 17. A surgical microscope including an interferometerarrangement comprising: a measuring beam path directed to and reflectedfrom an object; a reference beam path; means for interfering light ofsaid probe beam path with light of said reference beam path to generatean interference phenomenon pattern having a characteristic spatialfrequency k_(c); means for detecting said interference phenomenonpattern; and, said detecting means having a minimum spatial resolutionmatching said characteristic spatial frequency k_(c) of saidinterference phenomenon.
 18. The surgical microscope of claim 17,wherein said detecting means include a CCD-row having detector pixelshaving sizes and respective spacings therebetween so selected thatintensity variations of interference light, which intensity variationsoccur at the period of said characteristic frequency (k_(c)), cover atleast two of said detector pixels.
 19. A surgical microscope includingan interferometer arrangement comprising: a measuring beam path alongwhich a measuring beam is directed to and reflected from a region of anobject whereat scattering centers are exposed to said measuring beam; areference beam path; means for interfering light of said probe beam pathwith light of said reference beam path to generate an interferencephenomenon pattern having a characteristic spatial period; means fordetecting said interference phenomenon pattern; and, a signal evaluationunit coupled with said means for detecting said interference phenomenonpattern for determining a variation in time of said interferencephenomenon to permit precise measurement of movement of said scatteringcenters in the object region exposed to said measuring beam.
 20. Asurgical microscope including an interferometer arrangement comprising:a measuring beam path directed to and reflected from a specimen; a firstreference beam path; a second reference beam path; means for interferinglight of said probe beam path with light of said first reference beampath and said second reference beam path to generate an interferencephenomenon pattern having one or more characteristic spatialfrequencies; and, means for detecting said interference phenomenonpattern.
 21. An interferometer arrangement comprising: a measuring beampath directed to and reflected from an object; a reference beam path;means for interfering light of said measuring beam path with light ofsaid reference beam path to generate an interference phenomenon patternhaving a characteristic spatial frequency (k_(c)); and, means fordetecting said interference phenomenon pattern so as to provide aspatial resolution matching said characteristic spatial frequency(k_(c)) of said interference phenomenon causing said interferometerarrangement to have a relatively low signal to noise ratio and allowingmeasuring distances to be reliably based on as few as two pixels on saiddetecting means.